President Biden is expected to join European leaders in announcing new sanctions against Russia.

WASHINGTON — President Biden is expected to announce harsh new sanctions on Tuesday aimed at punishing Russia for what his top aides called the beginning of an invasion into Ukraine, joining a cascade of similar announcements by European leaders.

Mr. Biden is scheduled to speak Tuesday afternoon from the East Room of the White House less than a day after President Vladimir V. Putin of Russia issued a decree sending troops into two separatist-held regions in Ukraine. Russia’s Parliament authorized the use of military force abroad on Tuesday, taking another step toward what Western officials fear could be a full-scale assault against Ukraine.

The White House and world leaders condemned the move, calling it a clear violation of Ukraine’s borders.

But as the tough global response takes shape, Mr. Biden and his counterparts were struggling to balance the need to take swift and severe action while preserving the possibility of even greater sanctions on Russia if Mr. Putin escalates the conflict by attempting to seize the entire country — a war that could kill tens of thousands of people.

Germany announced Tuesday that it would halt certification of a natural gas pipeline linking it with Russia. The British government said that it would sanction members of the Russian Parliament who voted to recognize the independence of the separatist areas and would create legislation to ensure that no British individual or company could do business with the regions, Donetsk and Luhansk.

“This is the first tranche, the first barrage of what we are prepared to do,” Prime Minister Boris Johnson told Parliament on Tuesday.

In the United States, Jon Finer, Mr. Biden’s deputy national security adviser, said that Russia’s forces had begun to move into Ukraine, declaring on CNN that “an invasion is an invasion, and that is what is underway.” But he and Mr. Johnson shared the same sentiment, saying that “we’ve always envisioned waves of sanctions that would unfold over time in response to steps Russia actually takes.”

Two European officials said Tuesday that Russia had sent troops into the area, but Russia’s Foreign Ministry denied having done so.

Mr. Putin remained defiant in the face of the worldwide condemnations of his decision to recognize the independence of the so-called Donetsk and Luhansk People’s Republics created after Russia fomented a separatist war in eastern Ukraine in 2014.

In what sounded like an ultimatum, he demanded that Ukraine recognize Russia’s claim to Crimea and relinquish its advanced weapons.

A deputy defense minister, Nikolai Pankov, said that Ukraine had gathered 60,000 troops to attack the Russia-backed separatist enclaves in the country’s east — a step that Ukraine denies having any plans to take.

“Negotiations have reached a dead end,” Mr. Pankov said in a televised speech. “The Ukrainian leadership has taken the path of violence and bloodshed.”

There was no immediate sign of major military escalation in eastern Ukraine, but fearful Ukrainians boarded buses out of the separatist areas as Ukraine’s president, Volodymyr Zelensky, urged his beleaguered nation to “keep a cool head” in the crisis.

Mr. Zelensky insisted that Ukraine would not yield territory, and his defense minister, Oleksiy Reznikov, appeared to be girding his country’s troops for battle.

“Ahead will be a difficult trial,” Mr. Reznikov said in a somber message released by the military. “There will be losses. You will have to go through pain and overcome fear and despondency.”

A day earlier, Mr. Putin delivered a long, fiery speech that described Ukraine as part of Russia, calling the government in Kyiv little more than a “puppet” of the United States and its leaders solely responsible for whatever “bloodshed” may come next.

“As for those who captured and are holding on to power in Kyiv,” he said, referring to the Ukrainian capital, “we demand that they immediately cease military action.”